1. Field of the Invention
This invention relates to a control method and apparatus for optimizing the extraction of decomposition gas from solid waste landfills. More specifically, the present inventions pertains to a method using a wellhead device that when installed and operated on a landfill gas extraction well will provide the capability to select and maintain a constant flow rate of gas extraction from the subsurface. This capability allows controlling the extraction at near the gas generation rate for the well. The device also includes a high temperature shutdown feature in which the wellhead will stop gas extraction from the well for the purposes of minimizing the potential or exasperation of subsurface fires.
2. Description of Related Art
Once municipal solid waste is disposed of at a landfill, the organic fraction of the waste begins to decompose. This decomposition first proceeds through an aerobic biodegradation process where all the available oxygen in the buried waste is consumed. The decomposition then proceeds through a strictly anaerobic biodegradation process where the principle constituents of landfill gas are formed. Landfill gas consists of approximately 55% methane, 44% carbon dioxide and less than 1% trace gases. The trace gases consist of a wide variety of volatile organic compounds, which vary depending on the particular landfill. Noteworthy is the fact that oxygen is toxic to the microorganisms typically responsible for methane gas generation.
Since landfill gas is constantly being produced as a result of waste decomposition, landfill gas will move from the buried waste towards the ground surface and will result in surface emissions to the atmosphere. The constant generation of landfill gas also results in a flushing or purging action within the subsurface that results in the removal of air, thus further facilitating the anaerobic biodegradation process.
Surface emissions of landfill gas is not a desirable condition because the primary constituents of landfill gas are well known green house gases, which are thought to be contributing towards global warming. In addition, the trace gases present in landfill gas are believed to participate in an atmospheric photochemical reaction that leads to the formation of ozone, a principle constituent of smog.
In addition to surface emissions, landfill gas may also move or migrate laterally in the subsurface away from the buried decomposing waste, and may accumulate in near-by buildings or other structures. This condition creates a potentially dangerous condition due to the methane content of landfill gas. When methane is present in a concentration ranging from approximately 5 to 20 percent by volume it is potentially explosive. Another issue associated with subsurface migration of landfill gas is that it may also come into contact with groundwater and create the potential for groundwater contamination due to the presence of contaminating trace gases.
Thus it is desirable to collect landfill gas to prevent these negative environmental effects. It is also desirable to collect landfill gas for energy recovery purposes, as the methane content of landfill gas can be relatively easily used as a fuel.
Active landfill gas well extraction systems are used to control landfill gas surface emissions, control landfill gas subsurface migration away from the landfill, and often to collect landfill gas for energy recovery. These systems typically include an array of both vertical and horizontal landfill gas extraction wells that are in fluid communication with a common header piping system. The header piping system is, in turn, fluidly connected to a vacuum source, typically a centrifugal blower or other similar turbo-machine. Following extraction by the system, the gas may be incinerated by a flare, may be directly used as a fuel, or may be conditioned and then used as a fuel.
The landfill extraction system wells are either drilled or trenched into the landfill waste column and they consist of both perforated-piping sections and solid-piping sections. The solid piping section is nearest the surface of the landfill. The perforated-piping section is the deeper piping. The point at which the solid piping changes to perforated piping is a major design consideration for an extraction well, since it significantly influences the maximum allowable suction that can be applied to each well.
Each extraction well is in fluid communication with a header piping system through a wellhead assembly. The wellhead assembly typically consists of a gate valve used for throttling the volumetric flow rate of landfill gas from the extraction well and a sample collection port. The wellhead may also include a flow rate measurement device.
An operating goal of the landfill gas well extraction system is to remove gas at the approximate rate of its generation. The rational for this goal is the consequence of over- or under-extraction rates. Under-extraction rates mean the extraction rate is not high enough to prevent gas from reaching the surface or prevent subsurface migration. This results in air pollution, or a fire hazard. Over-extraction rates mean the extraction rate is high enough to draw large amounts of air into the waste column. This may cause a subsurface fire, and will kill many of the microorganisms responsible for the formation of methane, resulting in reduced methane recovery. Consequently, the gas flow rate from each individual extraction well, or group of adjacent wells, needs to be carefully monitored and controlled within a narrow operating range to prevent over- or under-extraction of landfill gas.
Several factors are involved in causing short-term over- or under-extraction rates in present extraction systems. Extraction systems typically rely upon one common vacuum source that is in fluid communication with the header piping system for all the wells. A centrifugal blower or similar turbo-machine is most commonly used as a vacuum source, although reciprocating compressors have also been used. The efficiency of turbo-machines is affected by gas temperature, which results in a variation in the mass flow rate developed by the turbo-machine with changes in temperature. This temperature-caused flow variation is reflected in a variation of the extraction rate at each of the system wells. The gas temperature changes with ambient temperature that, in turn varies over the course of a day, resulting in a diurnal variation. Landfill gas is alternately diluted and concentrated as a result of this mass flow rate variation. The magnitude of the effect depends upon the magnitude of the mass flow variation of the turbo-machine and the individual well construction characteristics.
Changing barometric pressures is another factor affecting extraction rates. Variations in atmospheric pressure modify the over-all resistance to flow in the extraction well and header piping system.
Changes in the landfill surface topology, such as differential settlement of the landfill surface, the development of landfill surface cracks, or waste separation from side slopes of the landfill may change the flow characteristics of the extraction system. These may cause a lower resistance to flow than an intact surface resulting in a short-circuit flow of air into the extraction system resulting in large quantities of air being drawn into the subsurface.
All of these effects result in a continuous cycle of over and under extraction of landfill gas by the extraction system. Methods to control these cycles have been proposed. U.S. Pat. No. 4,026,355 (Johnson et al.) describes a control method using various measurements of pressure within the landfill. This method has the drawback of requiring precise subterranean pressure measurements in the landfill that involve remote data communication. Such remote measurements in a landfill environment are subject to a variety of malfunctions from natural and man-made causes. U.S. Pat. No. 4,890,672 (Hall) describes a control method using measurement of the extracted gas temperature. This method is based on correcting an over-extraction by measuring its consequences, rather than controlling to prevent over-extraction. U.S. Pat. No. 6,169,962 (Brookshire et al.) describes a complex and extensive computer-controlled system and computer program involving using a wide variety of parameters to determine the control signal for the control valves in the extraction system. This system requires an expensive instrumentation and control system, and the communications for a remote control computer, which again is subject to malfunctions in a landfill environment.
What is needed is a simple, rugged, inexpensive control method and system that is based on the causes of landfill gas extraction system over- or under-extraction rates.